Conventionally, for example, a 4-tandem type color copying machine having four photoconductive drums (for yellow, magenta, cyan and black) horizontally arranged side by side is known as a color copying machine or more generally as an image forming apparatus. In such a color copying machine, for example, one polygon mirror is arranged between the second photoconductive drum and the third photoconductive drum.
Such a color copying machine has four light sources used for emitting laser beams towards the polygon mirror based on image signals for respective colors after color separation. For example, the four light sources are grouped into two sets and arranged on the left side and the right side with respect to the polygon mirror. The four light sources emit laser beams in different colors to respective surfaces of the photoconductive drums, which have been charged to a predetermined potential, to form electrostatic latent images thereon.
Furthermore, such a color copying machine has a plurality of mirrors between the polygon mirror and the respective photoconductive drums for guiding the laser beams having different colors. The plurality of mirrors and the polygon mirror constitute a scanning optical system. The scanning optical system includes a first scanning optical system that scans surfaces of the two photoconductive drums on the left side, and a second scanning optical system that scans surfaces of the two photoconductive drums on the right side.
The scanning optical system on the left side reflects the laser beams emitted from the two light sources on the left side on the same reflecting surface of the polygon mirror to deflect the laser beams in a main scanning direction and guides the laser beams via a plurality of mirrors to scan the surfaces of the two photoconductive drums on the left side. The scanning optical system on the right side reflects the laser beams emitted from the two light sources on the right side on another reflecting surface of the polygon mirror to deflect the laser beams in a main scanning direction and guides the laser beams via a plurality of mirrors to scan the surfaces of the two photoconductive drums on the right side.
Specifically, the respective laser beams emitted from the two light sources on the left side are reflected by the same reflecting surface of the polygon mirror. The reflected laser beams are deflected in the main scanning direction by the rotation of the polygon mirror and then are guided to the two photoconductive drums on the left side through the scanning optical system for corresponding colors, respectively. Similarly, the laser beams emitted from the two light sources on the right side are reflected by another reflecting surface of the polygon mirror. The reflected laser beams are deflected in the main scanning direction by the rotation of the polygon mirror and then are guided respectively to the other two photoconductive drums on the right side through the scanning optical system for corresponding colors.
The laser beams emitted respectively from the two light sources on the left side and the two light sources on the right side are incident on the polygon mirror with an opening angle in the main scanning direction. For example, in an apparatus disclosed in Japanese unexamined patent application publication No. 2008-122706, the laser beam emitted from each light source is incident on the polygon mirror without any angle in a sub-scanning direction. Therefore, in the apparatus disclosed in Japanese unexamined patent application publication No. 2008-122706, a change in an optical path due to unevenness of the surface has little influence.